In the fabrication of microcircuits, a light sensitive resist layer is typically exposed to a pattern of light formed by illuminating a patterned mask. The light sensitive layer is then developed to form a relief pattern corresponding to the mask pattern. The minimum size of the mask features which can be faithfully reproduced in the developed resist layer is directly related to the wavelength of the light used to illuminate the mask. Since it is desirable to be able to reproduce features as small as possible, the wavelength of the illumination being used has been constantly decreasing from the visible light region through the near ultraviolet range and into the deep ultraviolet range.
In order for an illumination system to be practical for such purposes, however, the spatial pattern of illumination intensity it provides at the mask plane must be substantially uniform. This uniform intensity also should be as high as possible so that an exposure can be made in as short a time as possible. In the deep ultraviolet range these two goals are not easily met simultaneously. The most intense deep ultraviolet light sources are excimer lasers, but the spatial light intensity from such sources is not sufficiently uniform. Furthermore, conventional methods for making the light from a non-uniform source more uniform tend to fail when applied to coherent laser light because coherent light tends to produce interference patterns of spatially varying light intensity.
One prior art approach to this problem has been to effectively move the light source during the exposure so as to smear or average the intensity pattern of the illumination. Another prior art technique for making illumination more uniform is to use an array of optical fibers to mix the spatial intensity pattern. However, these techniques are expensive to implement; much of the available light is lost during the mixing process; and the resulting illumination uniformity is not as good as is desired.